A rotation mechanism for a roll used to wind up film must be able to continuously rotate the roll at a constant rate so that the film will be rolled up tightly at a constant tensile force around an outer peripheral surface of the roll. When a gear-type reduction mechanism is used for the rotation mechanism, a vibration system including the rotation mechanism and the roll starts to vibrate due to angular transmission error in the rotation mechanism. When the resulting vibration matches an intrinsic vibration frequency of the vibration system, a state of resonance is created, large variation occurs in the rate at which the roll is rotated, and the film cannot be wound up in a suitable fashion.
In particular, when a wave gear reduction mechanism is used, a flexible external gear and a rigid internal gear are repeatedly engaged and disengaged with one another while portions of the flexible external gear are repeatedly flexed in a radial direction by the wave drive generator. For example, when the flexible external gear is flexed into an elliptical shape using an elliptical wave generator, the portions of the flexible external gear are flexed two cycles for every one rotation of the wave generator. Inherent rotational fluctuations are generated in the wave gear reduction mechanism due to such flexing.
In the roll rotation mechanism using the gear-type reduction mechanism, fluctuations in rate caused by angular transmission error in the reduction mechanism are magnified in the vibration system of the roll rotation mechanism, and the variations in rate are likely to increase. For this reason, the roll is not readily allowed to rotate continuously at a constant rate. Traction drive-type reduction mechanism, which have low rotational fluctuation, are used in the prior art for the roll rotation mechanism for allowing the roll to continuously rotate at a constant rate.
The invention disclosed in Patent Document 1 is an example of a roll rotation mechanism. The roller rotation mechanism disclosed therein has a configuration wherein the rate of rotation from a motor is reduced via a gear reduction mechanism composed of a pulley and a belt, and is transmitted to a roller rotation shaft via an Oldham coupling.
[Patent Document 1] JP-A 6-273766.
Traction drive-type reduction mechanisms are generally more expensive than gear-type reduction mechanisms. Belt/pulley-types cannot be mounted on a roll rotating shaft in a concentric state, and must have an installation space maintained at an external periphery.